An example of a prior art accelerometer design with high performance potential is described in U.S. Pat. No. 3,702,073. The accelerometer shown in that patent comprises three primary components, namely, a proof mass assembly and upper and lower stators between which the proof mass assembly is supported. The proof mass assembly includes a movable reed that is suspended via flexure elements to an outer annular support member. The reed and outer annular support member are commonly provided as a unitary structure composed of fused quartz.
Both upper and lower surfaces of the reed include capacitor plates and force restoring coils. Each force restoring coil is positioned on the reed such that the central axis of the coil coincides with a line that extends through the center of the reed and that is normal to the top and bottom surfaces of the reed. This line is coincident with the sensitive axis of the accelerometer. A plurality of mounting pads are formed at spaced apart positions around the upper and lower surfaces of the annular support ring. These mounting pads mate with inwardly facing surfaces of the upper and lower stators when the accelerometer is assembled.
Each stator is generally cylindrical, and has a bore provided in its inwardly facing surface. Contained within the bore is a permanent magnet. The bore and permanent magnet are configured such that an associated one of the force restoring coils of the proof mass assembly fits within the bore, with the permanent magnet being positioned within the cylindrical core of the force restoring coil. Current flowing through the coil therefore produces a magnetic field that interacts with the permanent magnet to produce a force on the reed. Also provided on the inwardly facing surfaces of the stators are capacitor plates configured to form capacitors with the upper and lower capacitor plates on the reed. Thus, movement of the reed with respect to the upper and lower stators results in a differential capacitance change.
In operation, the accelerometer is affixed to an object whose acceleration is to be measured. Acceleration of the object along the sensitive axis results in pendulous, rotational displacement of the reed and coils with respect to the support ring and the stators. The resulting differential capacitance change caused by this displacement is sensed by a suitable feedback circuit. The feedback circuit then produces a current that, when applied to the forces restoring coils, tends to return the reed to its neutral position. The magnitude of the current required to maintain the reed in its neutral position is directly related to the acceleration along the sensitive axis.
One of the most significant advantages of the accelerometer described above is that the reed, flexures and annular support member may be fabricated from a single piece of fused quartz, resulting in flexures with very high bias stability. A significant disadvantage of this accelerometer design is that the output is an analog signal. The accelerometer output signal must therefore be processed by a digital-to-analog converter or voltage-to-frequency converter prior to being used in a digital system. A comparatively new type of accelerometer that has an inherently digital output is the vibrating beam accelerometer, or VBA. The VBA is one member of a class of sensors that uses a force sensitive, crystal controlled oscillator as a force-to-frequency converter. In the case of the VBA, the force measured is the force required to accelerate a proof mass, and the oscillating crystal in a slender beam mechanically loaded along its longitudinal axis while being oscillated transversely in a beam bending mode. Just as the transverse component of restoring force in a guitar string will vary its frequency as a function of tension, so the axial force on an oscillating beam will vary its frequency as a function of tension or compression. The beam has a fixed frequency, determined by its mass and elastic properties, at which it oscillates under zero longitudinal load. Tension will increase that frequency, while compression will decrease it.
The VBA shares a design characteristic with all other sensors that depend on vibrating elements, in that spurious results can be obtained if significant amounts of vibratory energy are allowed to leave the system. In a VBA, the common soluton to this problem is to use a crystal that consists of two slender side-by-side beams, separated by a narrow central slot except at the beam ends wherein they merge into a common surface for attachment to associated structures. Gold electrodes are applied to the surfaces of the beams, such that the piezoelectric properties of the crystal material may be used to excite the twin beams into resonance in a transverse oscillation mode in which the two beams move in their common plane 180.degree. out of phase with one another. In this mode, the tight coupling between the end portions of the beams due to the very narrow slot causes them to resonante as a single structure at a single frequency, while the opposed stress fields associated with the bending moments at the ends of the beams merge and rapidly disappear, and are therefore not transmitted into surrounding structures.
All VBAs possess a number of significant advantages, including excellent scale factor stability. Many error sources can be greatly reduced by using two proof masses and two sensing crystals operated in a push-pull configuration, such that one crystal is put in compression while the other is put in tension, and treatng the output as some function of the frequency difference. This method of measurement cancels out many common mode errors, including the contribution of force crystal nonlinearity to the vibration rectification coefficient (VRC). However, a disadvantage of the use of dual proof masses is that identity of dynamic repsonse is difficult to achieve at frequencies approaching the sensor natural frequencies. An additional potential disadvantage of VBAs is that the proof mass requires caging to protect the crystal elements against shock overloads. Caging can be made difficult by full scale deflection on the order of 100 microinches. Shock caging is also sensitive to mismatches in coefficients of thermal expansion.
The crystal used as a force-to-frequency conversion element in a VBA can respond to forces only in one direction. This feature requires that the proof mass to which the crystal is attached have at least four of its six possible degrees of freedom relative to the instrument case constrained by some means. Although many constraints have been tried, it is generally considered that flexures provide the optimum means of proof mass constraint. In general, VBAs designed to date have made use of flexure that constrains the proof mass to a single degree of freedom, i.e., rotational motion about a hinge axis passing through the flexures. The force sensing crystal is attached to the proof mass, typically at its center of mass or at the end of the proof mass opposite the flexure, and extends in a direction normal to the hinge axis and to the pendulous axis. The force sensing crystal is therefore tangent to the circular arc to which the proof mass is constrained by the flexure.